Our present invention relates to a SQUID microscope for measuring or detecting magnetic characteristics of a sample and, more particularly to a method of and to an apparatus for determining magnetic properties of a sample utilizing a SQUID (superconductive quantum interference device).
T. J. Shaw et al, xe2x80x9cHigh-Tc SQUID Microscope Study of the Effects of Microstructure and Deformation on the Remanent Magnetization of Steelxe2x80x9d, IEEE Transactions on Applied Superconductivity, 1999 (9)2, pages 4107 to 4110 describe a SQUID microscope for determining magnetic characteristics of a sample. The SQUID is here mounted on a movable rod and the rod and SQUID are provided in a vacuum chamber which is coupled with a liquid nitrogen supply unit. A sample holder carrying the sample is disposed outside the vacuum chamber and a sapphire window separates the sample and the SQUID.
A drawback of this system is that the sample cannot be placed as close to the SQUID as might be desired. Also, in order to effect the measurement, a vacuum must be generated in the vacuum chamber.
From L. N. Vu et al, xe2x80x9cDesign and Implementation of a Scanning SQUID Microscopexe2x80x9d, IEEE Transactions on Applied Superconductivity, Volume 3(1), 1993, pages 1918 to 1921, a SQUID microscope is known. This microscope encompasses a chamber with liquid helium. Connected to this chamber is a sample chamber insulated by an intermediate vacuum layer. In the sample chamber, both the sample and the SQUID are provided, the SQUID being mounted on a movable rod. The rod can be inserted into and withdrawn from the sample chamber and can be sealed toward the exterior via a metallic bellows. The sample and SQUID are cooled by gaseous helium which is in a thermal connection with a liquid helium supply chamber. To cool the microscope, an outer cylinder cooled by liquid nitrogen is additionally provided. The insertion and sealing of the sample in this SQUID microscope is expensive and the need for recirculating gaseous helium is as a rule complex and costly.
It is the principal object of the present invention to provide an apparatus which is constituted as a SQUID microscope and which by comparison with earlier SQUID microscopes is simple and inexpensive and nevertheless can serve effectively for expelling magnetic properties of a sample.
It is also an object of this invention to provide a method of determining magnetic properties of a sample using a SQUID, whereby drawbacks of prior systems are avoided.
Still another object of the invention is to provide a SQUID microscope and a method of operating same whereby the spacing between the SQUID and the sample can be optionally small.
A further object of the invention is to provide a method of and an apparatus for determining magnetic properties of a sample which enables magnetic property measurement to be carried out at ambient pressure, i.e. without the need to evacuate the space containing the SQUID or sample.
These objects and others which will become apparent hereinafter are contained, in accordance with the invention in a method of measuring magnetic characteristics of a sample which comprises the steps of:
(a) cooling a SQUID to a superconducting temperature enabling the SQUID to respond to magnetic characteristics of a sample;
(b) juxtaposing the SQUID at superconducting temperature and the sample in a gaseous nitrogen atmosphere at ambient pressure; and
(c) effecting a measurement with the SQUID at superconducting temperature of a magnetic property of the sample in the gaseous nitrogen atmosphere at the ambient pressure.
The apparatus of the invention is thus a SQUID microscope which can comprise:
an upwardly open vessel;
a SQUID holder in the vessel configured to cool a SQUID to a superconducting temperature enabling the SQUID to respond to magnetic characteristics of a sample;
a SQUID mounted on the SQUID holder in the vessel and adapted to be cooled to the superconducting temperature enabling the SQUID to respond to magnetic characteristics of the sample; and
a sample holder in the vessel carrying a sample juxtaposed with the SQUID in a gaseous nitrogen atmosphere at ambient pressure whereby measurement of a magnetic property of the sample can be effected with the SQUID at superconducting temperature in the gaseous nitrogen atmosphere at the ambient pressure.
The SQUID is preferably cooled in step (a) with liquid nitrogen and the SQUID and sample can thus be provided in an upwardly open vessel containing this liquid nitrogen to a level just below the SQUID so that the SQUID and the sample are both disposed in the gaseous nitrogen atmosphere overlying the liquid level in this vessel. The vessel itself may be a cryostat and the cryostat can be provided with a lining of a xcexc metal shield.
The sample and hence the sample holder preferably is located above the SQUID and the SQUID holder can be a sapphire rod which is immersed in the liquid nitrogen.
More particularly, the method of the invention measures magnetic properties of a sample with a SQUID microscope in which the sample and the SQUID are both in a protective gas atmosphere of nitrogen at ambient pressure, i.e. the pressure of the atmosphere surrounding the apparatus and normally approximately atmospheric pressure. The protective gas atmosphere prevents or limits the condensing out or the freezing out, (sublimation) of undesired components on the surface of the sample and/or of the SQUID. Since the method operates at ambient pressure, expensive vacuum technology can be avoided. The sample change as a rule is simple and quick. The protective gas atmosphere can be supplied directly by the coolant both therebelow, the liquid nitrogen evaporating form its surface located preferably just below the SQUID.
The cooling of the SQUID with liquid nitrogen in the bath surrounding the rod on which the SQUID is mounted, greatly simplifies both the generation of the protective gas atmosphere from the coolant and the cooling action.
The use of an upwardly open vessel can communicate with the ambient atmosphere through the opening at the top of this vessel and which contains the liquid nitrogen amounts to a further simplification of the apparatus since it can ensure the ambient pressure for the SQUID and sample.
When the sample is located above the SQUID it can be raised or lowered and brought as close as may be desirable to the SQUID.
The vessel itself can be a Dewar flask or beaker composed of glass, stainless steel or other metals commonly in use with low-temperature systems and GFK.
The xcexc metal shield around the SQUID microscope of the invention reduces or eliminates distortion of the magnetic field applied to the SQUID and thus improves the measurement precision. The shield can form the opening communicating between the external atmosphere and the nitrogen blank in the vessel and this opening can be such as to just permit passage of the sample holder.
When the sapphire rod is used as a support for the SQUID, it can simultaneously serve as a heat transfer element between the liquid nitrogen bath and the SQUID. A sapphire rod has good thermal conductivity and very poor electrical conductivity.
The SQUID cooling can be independent from that of the sample and vice versa so that measurements can be taken even when the SQUID and the sample are at different temperatures.
When the gaseous nitrogen develops from the liquid nitrogen bath below the SQUID, it can serve to drive any moisture or air from the vessel and thus permit operation of the apparatus for long periods of time. The fact that moisture is driven out of the system prevents the SQUID, which is located above the liquid nitrogen level in the nitrogen atmosphere from icing up. The evaporated coolant thus forms the protective gas atmosphere.
Since the SQUID and the sample are not separated by a window in the SQUID microscope according to the invention, the sample can be practically brought into contact with the SQUID. A pressure sensor or contact sensor can be provided to output a signal upon contact of the sample with the SQUID and thus allow a process adjustment of the spacing between the SQUID and the sample.
The SQUID microscope of the invention has not only the advantage that it permits investigation of magnetic properties of the samples at ambient pressure, but also that expensive vacuum pumps and like technology can be avoided. Condensation or other deposition of undesired materials on the sample surfaces and/or on the SQUID are precluded and the freezing out of water on the sample surface is especially avoided.
Access to the SQUID or the sample for replacement or checking and even during the measurement (e.g. quality control) is ensured and the apparatus of the invention is especially characterized by permitting quick sample and SQUID replacement.
The distance between the sample and the SQUID can be as small as 1 xcexcm and, when the SQUID is a HTSL (high temperature superconductor) SQUID, the cooling is relatively simple and the SQUID and sample can be independently cooled or heated.